Electrophotography forms the technical basis for various well-known imaging processes, including photocopying and some forms of laser printing. One basic electrophotographic process involves placing a uniform electrostatic charge on a photoreceptor, and then exposing the photoreceptor to activating electromagnetic radiation in particular areas that correspond to an image to be printed or transferred. The electromagnetic radiation, which may also be referred to as “light”, may include infrared radiation, visible light, and ultraviolet radiation, for example. This exposure of the photoreceptor to light dissipates the charge in the exposed areas to form an electrostatic latent image. The resulting electrostatic latent image is developed with a toner, and then the toner image is transferred from the photoreceptor to a final substrate, such as paper, either by direct transfer or via an intermediate transfer material. The direct or intermediate transfer of an image typically occurs by one of the following two methods: elastomeric assist (also referred to herein as “adhesive transfer”) or electrostatic assist (also referred to herein as “electrostatic transfer”). Elastomeric assist or adhesive transfer refers generally to a process in which the transfer of an image is primarily caused by surface tension phenomena between a photoreceptor surface and a temporary carrier surface or medium for the toner. The effectiveness of such elastomeric assist or adhesive transfer is controlled by several variables including surface energy, temperature, pressure, and toner rheology. Electrostatic assist or electrostatic transfer refers generally to a process in which transfer of an image is primarily affected by electrostatic charges or charge differential phenomena between the receptor surface and the temporary carrier surface or medium for the toner. Electrostatic transfer, like adhesive transfer, is controlled by surface energy, temperature, and pressure, but the primary driving forces causing the toner image to be transferred to the final substrate are electrostatic forces. After the toned image is transferred by either type of transfer method, electrophotographic processes may further include the processes of fusing the transferred image to the substrate, cleaning the photoreceptor, and erasing any residual charge on the photoreceptor to prepare the system for the transfer of a new image.
In some common electrophotographic processes, the structure of a photoreceptor is a continuous belt, which can be supported and circulated by rollers or a rotatable drum, for example. Photoreceptors generally have a photoconductive layer that transports charge (either by an electron transfer or charge transfer mechanism) when the photoconductive layer is exposed to activating electromagnetic radiation or light. The photoconductive layer is generally affixed to an electroconductive support, such as a conductive drum or a substrate that is vapor coated with aluminum or another conductor. The surface of the photoreceptor can be either negatively or positively charged so that when activating electromagnetic radiation strikes certain regions of the photoconductive layer, charge is conducted through the photoreceptor to neutralize, dissipate or reduce the surface potential in those activated regions. An optional barrier layer may be used over the photoconductive layer to protect the photoconductive layer and thereby extend the service life of the photoconductive layer. Other layers, such as adhesive layers, priming layers, or charge injection blocking layers are also used in some photoreceptors. A release layer may also be used to facilitate transfer of the image from the photoreceptor to either the final substrate, such as paper, or to an intermediate transfer element.
Typically, a toner image that corresponds to the electrostatic latent image on the photoreceptor may be formed by providing a positively charged toner that is attracted to those areas of the photoreceptor that retain a less positive charge after exposure to electromagnetic radiation. Two commonly available general types of toners are referred to as dry toners and liquid toners. Dry toners will often be a powdered material comprising a blend or association of polymer and colored particulates, such as carbon for a black image, and liquid toners will often be a liquid material of finely divided solids dispersed in an insulating liquid that is frequently referred to as a carrier liquid. Generally, the carrier liquid may be a hydrocarbon that has a relatively low dielectric constant (e.g., less than 3) and a vapor pressure sufficiently high to ensure rapid evaporation of solvent following deposition of the toner onto a photoreceptor, transfer belt, and/or receptor sheet. Rapid evaporation is particularly important for cases in which multiple colors are sequentially deposited and/or transferred to form a single image.
Liquid toners can provide advantages over dry or powdered toners in certain applications because they are capable of producing higher resolution images while requiring lower energy for image fixing than dry toners. In addition, it is preferable for the toned image on the final substrate to be fixed to the substrate in such a way that it is resistant to removal in a variety of uses, abuses, and environmental conditions. However, the ink of the toned image that is deposited on the final substrate is often fragile and may not bear the attack of scratching or rubbing by outside forces such as human finger contact or such as erasure by a rubber pencil eraser, which may be referred to as poor “erasure resistance.” Furthermore, transferred inks having residual tack or stickiness may also undesirably stick to other final substrates when placed in a stack, which can cause image damage when adjacent substrates are separated from one another when a portion of the image peels away from the transferred image and onto another surface. This tendency of the image to undesirably transfer from one substrate to an adjacent substrate may be referred to as poor “blocking resistance.”
In order to render the inks to be adequately resistant to external forces such as blocking and erasure, it is sometimes desirable to heat the ink to an elevated temperature by contacting the surface of the final substrate to which the ink has been transferred with heat, such as a heated roll. Examples of fuser configurations having a single heated roller with at least one non-heated pressure roller for pressing a toned image toward the heated roller can be found in U.S. Pat. Nos. 4,806,097 (Palm et al.), 5,893,019 (Yoda et al.), and 5,897,294 (Yoda et al.). This process is commonly referred to as “fusing” and is often achieved by subjecting the final paper print to a heat source immediately after the transfer of ink to paper or another substrate. In the case of liquid toners, the use of heat can facilitate fixation of the ink by causing evaporation of the liquid portion of the toner. The heat also can serve to melt the toner particles onto the final substrate for permanence and durability.
Many types of heat sources may be used to fuse inks to paper or other mediums, such as a heated belt, a heated drum, or heated air, for example. Because some toners melt at different temperatures than others, the temperature necessary to adequately fuse the toner particles is usually customized to the chemical properties of the toner. If the temperature of the heating roller or element is too high, the toner may stick to the roller or other element and then be transferred back to the final substrate on a subsequent revolution of a roll, for example. This problem is known as “hot offset” and can often be cured by lowering the temperature of the roller. If the temperature of the heating roller or element is too cool, however, the toner particles may fail to fuse to the final substrate, and may also transfer to the roller or element, and possibly to the final substrate on a subsequent revolution, which may be referred to as “cold offset.” Thus, to achieve a proper transfer of toner in such a way that the ink can adequately bond to the final substrate, the heater roller or element should desirably be maintained at a relatively constant temperature within a defined range. This may be difficult to achieve, however, with certain types of heating systems.
Fusing images made with liquid toners thus presents special challenges as compared to the fusing of images created using other toner materials. First, the constant contact of liquid toner with a heated roller or element essentially creates a constant cooling “bath,” which may make it more difficult to maintain an adequate and relatively constant temperature for both eliminating the carrier liquid and fusing the image. Second, many of the devices and low surface energy materials used for dry toner fusing are not formulated to be used in a system where liquid or steam can penetrate, pool, run, or be imbibed, as is sometimes the case in electrophotographic systems using liquid toners. Third, traditional fusing, which is often used for dry toner systems where the final substrate is heated with the image facing the heating element, may not allow a sufficient amount of the evaporated carrier liquid to move away from the heating element, which may cause the carrier to undesirably re-condense on the final substrate and other components of the printing device. It is therefore desirable to provide devices, systems, and methods of fusing liquid toners that provide consistent, high quality images on a final substrate.